Microfluidic systems are being used increasingly in fields as varied as chemistry, biology, physics, analysis, diagnostics, screening etc. There are various types, and notably those employing a substrate serving as base or support. The substrates used are of massive material, selected from glass, silicon, metal, polymers, or a combination of all these materials.
In microfluidic systems of this type, microchannels may be engraved in the substrate by any known method. A component that is massive or a thin layer will then cover the substrate, thus delimiting the geometry of the microchannels. The microchannels may also be obtained by molding an elastomer in a suitable mold and then being arranged on a substrate. These microchannels may be arranged to form a network in which fluids circulate.
So as to be able to control the movements of the fluids, it is often advantageous to integrate valves and pumps in the microfluidic network.
However, to allow the integration of valves and pumps, a great many obstacles must be overcome. Moreover, the known valve systems have important limitations.
For example, in the process for fabrication of the valve systems, numerous steps are necessary, and they frequently require the use of special materials, which limits their applicability.
Another drawback of the known valve systems is their size, typically greater than 50 microns, which limits the number of them in a microfluidic system.
Various solutions have been proposed in the past in an attempt to solve these problems.
Thus, U.S. Pat. No. 6,408,878 proposes a method for fabricating a structure in molded elastomer to form microvalves used for closing or opening microchannels in a microfluidic system. The method comprises the following steps:                forming a first layer of elastomer above a first micromachined mold, this first mold having a protuberance on its upper face which creates a recess on the bottom surface of said first layer of elastomer,        forming a second layer of elastomer above a second micromachined mold, this second mold having a protuberance on its upper face which creates a recess on the bottom surface of said second layer of elastomer,        applying the bottom surface of the second layer of elastomer on the top surface of the first layer of elastomer so that a first channel is formed in the recess between the first and the second layer of elastomer,        positioning the first layer of elastomer above a flat substrate in such a way that a second fluidic channel is created in the recess between the first layer of elastomer and the substrate.        
In this way, by applying a pressure in the first channel by means of a liquid or gaseous fluid, the top surface of the first layer is deformed and the second fluidic channel is thus closed.
These devices do, however, have many drawbacks, and in particular:                they require external circuits, comprising tubes and valves, for supplying and controlling the pressures in the microfluidic control channels. These external circuits become more complex as the number of valves integrated in the microfluidic system increases;        fabrication of submicrometric valves according to the method is long and expensive. Such fabrication requires masks of submicrometric precision and extremely thin elastomer membranes. In fact, it is the deformation of this membrane within the channel to be controlled that provides its closure or opening. Now, closure is only effected if the deformed membrane will completely obstruct the cross section of the microchannel in question. This requires very small thicknesses relative to the dimensions of the microchannel cross section and therefore great fragility. Moreover, control of the pressure of the fluid in the first channel must be very low and the control of this pressure must be very precise.        finally, the method requires the use of an elastomer as material, which limits its fields of application.        
U.S. Pat. No. 6,488,872 also proposes a method for fabrication of microvalves used for closing or opening microchannels in a microfluidic system. The method comprises the following steps:                in a preliminary step, pillars are positioned in the microchannels,        the microchannels are filled with a solution of photocrosslinkable monomers,        through a mask, obtained for example by a photolithographic process, said solution of monomers is submitted to UV radiation around the pillars. Under the effect of this radiation, polymerization/crosslinking of the monomers in solution takes place around the pillars, which then become coated with a solid polymer layer. A monomer is selected such that said solid layer has the property of undergoing a volume change in is the presence of a stimulus (for example a change in pH or temperature of the fluid around said pillars).        
Thus, to operate the microvalves, the stimulus in question is applied at the level of the pillars, which has the effect of swelling the layer until the microsystem is blocked.
These devices also have many drawbacks and in particular:                fabrication requires several steps: filling, photopolymerization, rinsing,        the photopolymerization step requires micrometric precision for aligning the mask with the pillars. Moreover, the optical system providing illumination must be able to avoid phenomena of reflection and diffusion of UV radiation. In fact, such phenomena would have a destructive effect, causing polymerizations in undesirable places, which would have the effect of sealing the microchannels in said undesirable places,        the adhesion of the polymers on the pillars is weak and uncontrolled, which leads to many drawbacks.        
U.S. Pat. No. 7,988,685, like U.S. Pat. No. 6,408,878, employs two networks of microchannels separated by a membrane, but control is provided not by a change of pressure in one of the two networks but by the swelling caused by a hydrogel present in the control microchannel, which deforms the membrane and will close the channel.